The present invention relates to an electroosmotic dewaterer for continuously extracting water from a substance to be dehydrated, including surplus liquid mud produced in sewage disposal plants and mud slurries produced in the fields of food and other industries, by the use of electroosmotic effect and pressure.
Various types of conventional electroosmotic dewaterer is disclosed in Japanese Patent Application Laid Open No. 25597/85. Referring to FIG. 1 through 3, a description will be given to the schematic construction of an electroosmotic dewaterer of conventional rotary drum type and continuous process type. As shown in FIG. 1, a rotary drum 1 is provided with an anode electrode 2 made of high electrolytic corrosion resistant material and bonded to the outer peripheral face thereof. The drum shaft 1a of the rotary drum 1 is pivotally supported by a frame 4 shown in FIG. 2 via a bearing 3 as shown in FIG. 2. An endless filter belt 5 and a metal caterpillar-like press conveyor 6 used as a cathode electrode and superposed on the filter belt 5 is stretched on sprockets 7a.about.7d opposite to a part of peripheral area of the rotary drum 1. A liquid mud compression passage 8, whose gap between the electrodes gradually decreases from entrance toward exit, is provided between the filter belt 5 and rotary drum 1. Further, a filtered water receiving saucer 9 communicating with a discharge channel outside the system is installed via the filter belt 5 and the press conveyor 6 under the liquid mud compression passage 8.
A driving motor 10 is interlockingly coupled to the sprocket 7b and a liquid mud supply hopper 11 is arranged at the entrance of the liquid mud compression passage 8, whereas a scraper 12 for peeling and recovering the dehydrated mud cake is set opposite to the filter belt 5 at the exit thereof. Further, presses 13, together with press position adjusting bolts 14 and hydraulic cylinders 15, are installed along the peripheral face of the press conveyor 6 at the back of the liquid mud compression passage 8.
On the other hand, a d.c. power supply 18 is connected via a power supply brush 17 to a power supply slip ring 16 attached to the shaft end 1a of the rotary drum 1 and to a slip ring installed at the supporting shaft of the sprocket 7a being in contact with the press conveyor 6 of a cathode electrode. In this case, the anode electrode 2 arranged on the periphery of the rotary drum 1 is provided with the following insulating construction so as to be supplied with power from the power supply 18. As shown in FIGS. 2 and 3, the anode electrode 2 is installed via an insulating material 19 on the periphery of the rotary drum 1 and both edges thereof are fixed to the rotary drum 1 by end plates 20 of fiber reinforced plastics. Further, a bus bar 21 led from the slip ring 16 provided at the shaft end via the hollow drum shaft 1a into the rotary drum 1 is connected via a connecting bolt 22 passed through the rotary drum 1 to the electrode 2. In order to insulate the connecting bolt 22 relative to the rotary drum 1, the connecting bolt 22 is provided with an insulating washer 23 and sleeve 24. The drum shaft 1a of the rotary drum 1 is divided in the axial direction to back up the insulation thereof and insulating materials 25 are held between flanges attached to the shaft thus divided, between the drum shaft and the slip ring 16 and between the bearing 3 and the frame 4 installed on the ground side, respectively. The insulating materials 25 are insulated and bolted in the same manner as what is shown in FIG. 3. Such power supply system to the anode electrode and particularly a power supply construction toward the anode electrode attached to the rotary drum is also disclosed in Japanese Patent Application Laid Open No. 60604/81 and is well known.
With the aforesaid arrangement, when liquid mud 26 as a substance to be dehydrated is supplied via the hopper 11 into the liquid mud compression passage 8 while the driving motor 10 is operated and the d.c. voltage is applied by the power supply 18 to the anode and cathode electrodes, the liquid mud is carried toward the exit of the liquid mud compression passage, i.e., in the direction of arrow P while being sandwiched by the rotary drum 1 and the filter belt 5. During the carrying step, electroosmotic action, in addition to mechanical compressive force, is applied by the electric field formed between the facing electrodes to the liquid mud 26 being carried. The slurry particles of the liquid mud 26 is forced by the electroosmotic action to move to the anode side, whereas the water included in the liquid mud 26 is positively charged, caused to flow toward the cathode side and discharged thereat, then by the aforesaid mechanical compressive force in addition to the electroosmotic action, the water passes through the filter belt 5 and is brought down via the discharged channel formed in the press conveyor 6 to the filtered water receiving saucer 9 and discharged therefrom. On the other hand, the liquid mud 26 thus dehydrated in the passage 8 is converted into a cake containing a low percentage of water and the cake 27 thus dehydrated is discharged via the scraper 12 from the exit of the passage 8 and burned or reutilized as a composite fertilizer.
The following problems arises when the electroosmotic dewaterer thus constructed is operated to dehydrate sewage mud and the like.
When the aforesaid electroosmotic dewaterer was used to dehydrate mud produced in a sewage plant, white solid substances were deposited on the surface of the press conveyor of cathode electrode as the operating time lapses and those substances were seen to have accumulated in the form of layer. As the quantity of the solid substances thus accumulated increased, the power consumption of the electroosmotic dewaterer increased and the discharge channel of the press conveyor was also clogged with the deposits, thus preventing the smooth flow of the filtered water. In consequence, it was confirmed that the dehydration efficiency was reduced to a considerable extent.
The present inventors therefore collected the white deposits and made a quantitative analysis of them using an X-ray analyzer. The table 1 below shows the results of the quantitative analysis.
TABLE 1 ______________________________________ Quantity Quantity Components (weight %) Components (weight %) ______________________________________ Ca ++++ Zr tr.about..+-. K ++ Sr .+-..about.+ Cl +.about.++ Zn tr.about..+-. Si ++.about.+++ Cr tr.about..+-. Al + Ni tr S ++ Fe .+-..about.+ P ++ Mn .+-..about.+ Mg ++.about.+++ Cr tr Na tr ______________________________________ where the weight % of the standard quantity in the above table is: ++++ . . . more than 10; +++ . . . 1.about.10; ++ . . . 0.1.about.1; + . . . 0.01.about.0.1; .+-. . . . 0.001.about.0.01; and tr . . . less than 0.001.
In the X-ray diffraction analysis of the deposits, the calcium content as the largest one in the above table was proved to be calcium hydroxide. The reason for the production of calcium hydroxide during the electroosmotic is assumed to be the following; namely, the liquid mud in the sewage contains a number of calcium ions, which are positively charged cations and, when the liquid mud is electroosmotically dehydrated, the calcium ions together with the water contained in the mud are led to the cathode side, whereby the calcium hydroxide is produced in accordance with the reaction expressed by the following equation and attached to the surface of the cathode electrode: EQU Ca.sup.+ +2e.fwdarw.Ca, Ca+2H.sub.2 O.fwdarw.Ca(OH)
Moreover, the calcium hydroxide is, as is well known, electrically nonconductive and such a substance accumulated and attached to the surface of the cathode electrode increases the resistance of the electrode and power loss when the voltage is applied thereto and thus causes the electrode to produce a large amount of Joule heat.
In the meantime, the speeds at which the solid substances attach to the electrode and the electric resistance of the electrode increases, were examined on the basis of the results obtained from the test machine and the present inventors observed:
speed at which the solid substance attached to the electrode=0.01 g/AH.multidot.dm.sup.2 ; and speed at which the electric resistance increased=0.04 .OMEGA./AH.multidot.dm.sup.2. In the process of electroosmotical dehydration of mud, voltage is applied across the anode and cathode electrodes so that a current having a density ranging from 3 to 6 A/dm.sup.2 is normally caused to flow therebetween. Accordingly, the quantity of substances produced and attached to the surface of the press conveyor as a cathode electrode and that of increased electric resistance per unit time when the electroosmotic dewaterer is operated to dehydrate the sewage mud, can be obtained by multiplying the current density by the speeds at which the solid substance attaches thereto and at which the electric resistance increases. The values thus obtained by way of trial are:
Quantity of substances attached=0.3.about.0.6 g/dm.sup.2
Quantity of increased electric resistance=0.12.about.0.24 .OMEGA./dm.sup.2
Further, the following problems as to the maintenance of insulation of an electrical path to the anode arises during the operation of the electroosmotic dewaterer thus constructed.
When the aforesaid electroosmotic dewaterer is started, a large quantity of Joule heat is generated as the liquid mud is supplied with power so that a large quantity of steam is produced from the heated liquid mud and is spread in the surroundings. Because of this, in the conventional power supply construction relative to the anode electrode 2 permits the steam enters into the small gap between the insulating materials installed in each principal places and condenses therein, so that defective insulation occurs between the electrode 2 and the rotary drum 1 or the rotary drum 1 and the frame 4. Moreover, because 3.about.5 kg/cm.sup.2 pressure is applied to the liquid mud in the passage 8 during the operation of the dewaterer, the reaction force of the compression force is applied to the rotary drum 1 as a bending stress. For the above reason, the drum shaft may be bent and the insulating material inserted in between the flanges of the divided shafts may often be damaged and, when the insulating material is thus damaged, the steam is facilitated to enter into the small gap therebetween. In consequence, the defective insulation is often caused during the operation of the conventional electroosmotic dewaterer, whereby power often leaks from the anode via the rotary drum, the frame and the like to the ground, so that the trouble of the loss of electroosmotic dewaterer function often occurs. In order to avoid the operating trouble of the dewaterer due to the defective insulation, the dewaterer should be subject to frequent checks for insulation and, when a defective insulation point is found, the defective insulation point should be remedied correctly each time it is found. Thus, the management of dewaterer operation is troublesome.
On the other hand, a measure to prevent the generation of the defective insulation has once been taken by completely covering the gap between the insulating points with resin. However, the increased size of the machinery limits the application of the aforesaid method and the only method has been to coat the insulating points with resin. According to the resin coating method, the layer of resin is thin and to completely cover the gap with the layer of resin is difficult so that sufficient reliability can not be attained.
Still further, the following problems as to the anode electrode arise. The anode electrode 2 of the rotary drum for use in the aforesaid electroosmotic dewaterer has conventionally been made of stainless steel, nickel steel or soft-iron in the form of a plate, or carbon-sintered plate. According to the facts known to the present inventors through the operation of such an electroosmotic dewaterer, the anode electrode made of the aforesaid materials has the following disadvantages: namely, the electrode made of stainless, nickel steel or soft-iron allows its component to ionize because of the current flowed and to melt into the liquid mud so that the electrode is consumed as the time elapses. The value of the consumption per unit time under a current supply condition of 1 A/dm.sup.2 in the case of stainless steel (SUS 304) is 156 mg/A-Hr and as great as 1,050 mg/A-Hr in the case of the nickel steel. Furthermore, the life of the electrode is shortened because a current of about 3 A/dm.sup.2 practically passes through such an electroosmotic dewaterer. Moreover, the electrode made of stainless steel, nickel steel or the like may cause a secondary pollution because the heavy metal ions melted out of the electrode supplied with current is mixed into the substance to be dehydrate or filtered water. In addition, the soft-iron plate may be oxidized during the use and oxidized iron is formed in layers on the electrode, which extremely decreases conductivity thereof. When precious metals such as platinum are used for the anode electrode, the consumption of the electrode is small and excellent conductivity is attained, however, such metals are expensive and cannot be put to practical use economically. On the other hand, the carbon sintered plate has a feature that its consumption due to the flow of current is 47.6 mg/A-Hr under the same condition, which is considerably smaller than that of the aforesaid metal electrode. However, its electrical resistivity is large so that current flow property is low. Also, its mechanical strength and wear resistance is low so that cracks may be produced because of the compressive load applied to the electrode in use, and the wear thereof resulting from its contact with the substance to be dehydrated may be accelerated.
If the facing electrodes are worn out during the operation of the electroosmotic dewaterer, the distance therebetween slightly changes and this fact makes it necessary to alter the voltage supply condition of the power supply and to adjust the distance therebetween to maintain the efficient operation of the dewaterer. In order to cope with the aforesaid situation, the worn out electrodes must be replaced with new ones in a relatively brief period with interruption of the operation thereof. The resulting trouble of replacing the electrode and the time required therefor have reduced the operating efficiency of the dewaterer. The properties of the electrode, particularly the durability thereof form an important part in maintaining the performance of the dewaterer and, in view of this, the selection and improvement of materials constituting the electrode is an important problem.